SIZING ACCURACY, COUNTING EFFICIENCY, LOWER DETECTION LIMIT AND REPEATABILITY OF A WAFER SURFACE SCANNER FOR IDEAL AND REAL-WORLD PARTICLES

The performance characteristics of a Tencor Surfscan 4000 wafer surface scanner have been evaluated using ideal polystyrene latex (PSL) spheres and irregularly shaped real-world particles of Si and SiO2. The particles were uniform in size and were deposited on bare silicon wafers and used as standard calibration wafers to study the scanner response. Particles in the 0.1 to 1.0 mum diam range were used. The sizing accuracy, counting efficiency, lower detection limit, and count repeatability of the scanner were studied systematically. The full Maxwell's electromagnetic equations also have been solved numerically on a supercomputer to obtain the light-scattering cross section of the particles on bare silicon wafers. The calculation compares favorably with the experimental results. Because of the high refractive index of silicon, the wafer surf ace scanner detects Si particles to a considerably smaller size than PSL. The data suggest that Si particles as small as 0.1 mum have been detected by the Surfscan 4000 with 90% counting efficiency even though the nominal lower limit of the instrument is 0.3 mum based on PSL calibration.